1. Field of the Invention
The present invention relates to an image heating apparatus that is represented by a heat fixing device mounted on an image forming apparatus such as a copying machine or a printer, a device that heats an image on a recording sheet to improve the surface property of the image, and the like.
2. Description of the Related Art
For example, in an electrophotographic image forming apparatus of a transfer system which is equipped with the above-mentioned image heating apparatus as a toner image heat fixing device for the recording sheet onto which a toner image has been transferred, if a trouble such as jamming occurs downstream of a nipping portion to thereby shut out the transportation of the recording sheet, the recording sheet is not transported anywhere, and the recording sheet is caught by a heating unit, resulting in a state where a jam clearance is difficult.
For that reason, in order to solve the above problem, up to now, various techniques have been disclosed, for example, in Japanese Patent Application Laid-open No. 10-74015, Japanese Patent Application Laid-open No. 9-236958, and the like.
Japanese Patent Application Laid-open No. 10-74015 discloses a fixing device for use in an image forming apparatus that is characterized in that a separation claw is always urged by a spring toward a direction along which the separation claw is in press contact with a fixing roller that functions as a heating unit, and when the separation claw is moved in a fixing roller outer peripheral direction by a jammed sheet, the movement of the separation claw is detected by a sensor to detect that the recording sheet is caught by the fixing roller.
Also, Japanese Patent Application Laid-open No. 9-236958 discloses an image forming apparatus that is characterized in that twine of the recording sheet around the heating roller is detected by a torque value that is detected by a torque detecting means that detects a load torque of the heating roller which is disposed between a drive device and the heating roller that functions as a heating unit.
FIG. 14 is a schematic structural view showing an example of an image heating apparatus, which employs a film heating method and a pressure roller drive method and is a tensionless type using a cylindrical thin heat-resistant film. This image heating apparatus is disclosed in, for example, Japanese Patent Application Laid-open Nos. 4-44075 and 4-44033 and the like.
A heating unit 1 is a laterally elongated member that is longitudinal in a direction vertical to a drawing plane of FIG. 1. The heating unit 1 is made up of a stay 6 that is of the semi-arc trough shape in a lateral cross-section and has a heat resistance and rigidity; a plane shaped heating member (for example, a ceramic heater) H that is laterally elongated, thin and has a low heat capacity which is fixingly fitted into a seat groove portion provided in a lower surface of the stay 6 along the longitudinal direction of the stay; a thin heat-resistant film 5 that is loosely externally fitted onto the stay 6 to which the heating member H is attached; a rigid pressure member 11 a lateral section of which is of a U-shape that is directed downward which is inserted into the stay 6, and the like.
The pressure roller unit 2 is made up of a core 2a and a heat-resistant elastic material layer 2b that is coaxially molded on the outer periphery of the core 2a into a roller shape. The pressure roller unit 2 is disposed so that both end portions of the core 2a are rotatably held between side plates (not shown) at front and back sides of a device chassis 12 through bearings (not shown).
The heating unit 1 is disposed on the upper side of the pressure roller unit 2 in parallel with the pressure roller unit 2 in such a manner that the heating member R side is directed downward, and both end portions of the rigid pressure member 11 are urged against the pressure roller unit 2 by a pressure urging member (not shown), to thereby bring the downwardly directed surface of the heating member H in pressure contact with the heat-resistant elastic material layer 2b of the pressure roller unit 2 through the thin heat-resistant film 5 against the elasticity of the elastic material layer under a given pressure force to form a nip portion N with a given width as a heating portion.
The pressure roller unit 2 is rotationally driven at a given peripheral speed in a counterclockwise direction xe2x80x9caxe2x80x9d indicated by an arrow by a drive means M. A rotating force is exerted on a cylindrical thin heat-resistant film 5 due to a pressure contact frictional force at the nip portion N between the outer surface of the pressure roller unit 2 and the thin heat-resistant film 5 caused by the rotation of the pressure roller unit 2. The thin heat-resistant film 5 is rotated in a clockwise direction axe2x80x2 indicated by an arrow on the outer periphery of the stay 6 while the inner surface of the thin heat-resistant film 5 is slid in close contact with the downwardly directed surface of the heating member H.
The pressure roller unit 2 is rotationally driven with the state where the cylindrical thin heat-resistant film 5 is rotated by the rotation of the pressure roller unit 2. Also, the heating member H is energized, and the temperature of the heating member H rises and is adjusted up to a given temperature. In such states, a recording sheet S that bears an unfixed toner image T is introduced between the thin heat-resistant film 5 and the pressure roller unit 2 in the nip portion N, and the recording sheet S is nipped and transported together with the thin heat-resistant film 5 in a state where the toner image bearing surface side of the recording sheet S is in close contact with the outer surface of the thin heat-resistant film 5 at the fixing nip portion N. In the nipping and transporting process, the heat of the heating member H is imparted to the recording sheet through the thin heat-resistant film 5, and the unfixed toner image T on the recording sheet S is heated and pressurized on the recording sheet S to make fusion fixation Txe2x80x2.
The stay 6 of the heating unit 1 is equipped with a semi-circular separation curved portion 9 that is formed continuously in the widthwise direction of the recording sheet S downstream of the nip portion N in the thin heat-resistant film rotation direction, and the curvature of the thin heat-resistant film 5 is partially made large, to thereby self-strip the recording sheet S that has passed the nip portion N from the thin heat-resistant film 5.
The recording sheet S that has been self-stripped from the thin heat-resistant film 5 passes through upper and lower guide plates 13 and relayed by a pair of delivery rollers 4 so as to be transported and delivered.
Also, because the heating unit 1 and the pressure roller unit 2 become high in temperature, in order to prevent a user from directly touching those units 1 and 2, there is provided a fixing cover 3 that covers the heating unit 1 and the pressure roller unit 2.
In the above-mentioned image heating apparatus, as shown in FIG. 15, when jamming occurs due to some cause, and the recording sheet S becomes in an accordion state inside of the fixing cover 3, there is no space into which the trailing end of the recording sheet S is inserted, resulting in a fear such that the thin heat-resistant film 5 rotates in such a state where the recording sheet S and the thin heat-resistant film 5 are in close contact with each other through the toner and the recording sheet is inserted into the nip portion N again. As a result, the trailing end of the recording sheet S and a portion of the recording sheet which has been inserted into the nip portion N again are allowed to adhere to each other with toner T, and the jam clearance becomes difficult.
In order to overcome the above drawback, as shown in FIG. 17, a rib R is disposed on a wall of the fixing cover 3 downstream of the heating unit 1, and a clearance xe2x80x9ctxe2x80x9d N between the rib R and the heating unit 1 is narrowed to prevent the twine of the recording sheet S around the thin heat-resistant film 5.
The above-mentioned respective conventional arts have the effect of preventing the recording sheet S from being twined around the heating unit 1, but are desired to improve the following matters.
That is, the device disclosed in Japanese Patent Application Laid-open No. 10-74015 requires a shaft and a lever member for the spring for press-contacting the separation claw to the fixing roller that functions as the heating unit and the twine detection as described above, and therefore becomes complicated in its structure. Also, because a force is always applied to the fixing roller in a direction along which the separation claw is in press contact with the fixing roller, a slight amount of toner that is stuck onto the fixing roller is stuck onto the extremity of the separation claw and laminated. If the laminated toner is stuck onto the fixing roller again, an image stain occurs on the recording sheet, or an expensive material to which the toner is not stuck is required to be used. Also, because the separation claw is always urged against the fixing roller, a specific and expensive material that is high in heat resistance and prevents the fixing roller from being damaged must be selected. In addition, because the separation claw is so structured as to always urge the separation claw against the fixing roller, there arises such a problem that the fixing roller is liable to be damaged in assembling.
Also, the device disclosed in Japanese Patent Application Laid-open No. 9-236958 detects the twine of the recording sheet on the basis of a torque value detected by the torque detecting means that detects the load torque of the heating roller which is disposed between the driving device and the heating roller that functions as the heating unit as described above. If the heating roller is made small in diameter for the efficiency of the heat transfer and also made high in speed, even if the operation of the drive device stops after the detection of the torque, the recording sheet is conveyed by the inertia of the heating roller or the like, and the jam clearance of the recording sheet twined around the heating roller becomes difficult.
Further, in the image heating apparatus of the film heating system shown in FIG. 17, even if the clearance t is set to be narrow as described above, when the printing density on the recording sheet S is high, there is a fear that the thin heat-resistant film 5 and the recording sheet S are brought in close contact with each other through the toner, the thin heat-resistant film is pushed against the recording sheet S, the clearance t between the recording sheet S and the fixing cover 3 is widened, the recording sheet S passes through the clearance xe2x80x9ctxe2x80x9d, and the recording sheet S is wound on the thin heat-resistant film 5.
The present invention has been made in view of the above-mentioned problems, and therefore an object of the present invention is to provide an image heating apparatus that is capable of preventing a recording material from being twined around a rotary member.
Another object of the present invention is to provide an image heating apparatus that is capable of preventing the recording material from being twined around the rotary member while a damage on the rotary member is reduced.
Still another object of the present invention is to provide an image heating apparatus that readily conducts a jam clearance after the recording material has been jammed.
In order to achieve the above objects, according to the present invention, there is provided an image heating apparatus, comprising:
heating means having a nip that nips and transports a recording material that bears an image, the heating means having a rotary member that is in contact with the recording material; and
a moving member disposed downstream of the nip in a moving direction of the recording material so as to be movable between a first position in which the moving member abuts against the rotary member and a second position in which the moving member is apart from the rotary member,
wherein the moving member is positioned in the second position when the recording material is normally transported, and the moving member moves to the first position when the recording material is abnormally transported downstream of the nip.
Other objects of the present invention will become apparent by reading the following detailed description with reference to the accompanying drawings.